Inductance variable device

ABSTRACT

A inductance variable device includes a first inductor, a second inductor magnetically coupled to the first inductor, a current source whose current is variable, electrically connected to the first inductor, and a current control unit which controls a current flowing from the current source to the first inductor according to a feedback signal having frequency information in a current flowing in the second inductor to vary a combined inductance of the second inductor.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to an inductance variable device in whichinductance can be varied.

2. Background Art

An inductor device in which inductance can be varied by electrical meansis used for such as a resonance circuit in which a resonance frequencycan be varied over a large frequency range. In the description below, anelement having an inductance such as a coil is referred to as an“inductor”, and a device comprising inductors is referred to as an“inductor device”.

An example of conventional inductor devices is disclosed in JapanesePatent Laid-open Publication No. 2002-151953. This inductor device willbe described with reference to FIGS. 8 to 10.

An inductor device 40 shown in FIG. 8 includes a square frame-shapedmain inductor 41 formed of a conductor on an insulation substrate (notshown), and a sub inductor 42 formed outside the main inductor 41 andhaving a square frame shape larger than that of the main inductor 41. Apart of the frame of the main inductor 41 is cut so that terminals 41 aand 41 b are led to the outside. In addition, a part of the frame of thesub inductor 42 is cut so that terminals 42 a and 42 b are formed. Afirst switch 44 serving as a semiconductor switch is connected to theterminals 42 a and 42 b. In addition, a second switch 46 is connectedbetween the terminal 42 b and a circuit ground G.

When an alternate current is applied to the main inductor 41 and thefirst switch 44 is closed to short-circuit the terminals 42 a and 42 b,inductance of the main inductor 41 is varied by magnetic coupling. Whenthe first switch 44 is closed, the second switch 46 is opened. When thefirst switch 44 is opened, the second switch 46 is closed and the subinductor 42 is grounded.

According to another conventional inductor device 50 shown in FIG. 9,inside a square frame-shaped main inductor 51, a square frame-shaped subinductor 52 which is smaller than the main inductor 51 is provided. Apart of the frame of the main inductor 51 is cut so that terminals 51 aand 51 b are led to the outside. In addition, a part of the frame of thesub inductor 52 is cut so that terminals 52 a and 52 b are provided, anda first switch 53 is connected between the terminals 52 a and 52 b. Inaddition, a second switch 54 is connected between the terminal 52 b anda circuit ground G. When an alternate current is applied to the maininductor 51 and the first switch 53 is closed to short-circuit theterminals 52 a and 52 b, inductance of the main inductor 51 is varied bymagnetic coupling. When the first switch 53 is closed, the second switch54 is opened. When the first switch 53 is opened, the second switch 54is closed and the sub inductor 52 is grounded.

According to still another inductor device 60 shown in FIG. 10, a spiralshaped sub inductor 62 is formed so as to overlap with a spiral maininductor 61. A first switch 63 is connected between terminals 62 a and62 b of the sub inductor 62. In addition, a second switch 64 isconnected to the terminal 62 b and a circuit ground. When an alternatecurrent is applied to the main inductor 61 and the first switch 63 isclosed to short-circuit the terminals 62 a and 62 b, inductance of themain inductor 61 is varied by magnetic coupling. When the first switch63 is closed, the second switch 64 is opened. When the first switch 63is opened, the second switch 64 is closed and the sub inductor isgrounded. According to the constitutions shown in FIGS. 8 to 10, theinductance of each of the inductor devices 40, 50 and 60 can be variedby increasing or decreasing the number of spirals of the main inductor.

In addition, Japanese Patent Laid-open Publication No. 2002-9544discloses a voltage control oscillator including an inductor for theoscillator and a control inductor. According to this voltage controloscillator, a current to be applied to the control inductor is set to apredetermined value and a predetermined mutual inductance is generatedin the inductor for the oscillator.

SUMMARY

According to the conventional inductor devices 40, 50 and 60 shown inFIGS. 8 to 10, the inductance of the main inductor can be varied byopening or closing the first switch. Since the inductance is varied bythe switch, the inductance is varied between two inductance values andthe inductance cannot be varied sequentially.

In addition, according to the voltage control oscillator described inthe Japanese Patent Laid-open Publication No. 2002-9544, since apredetermined current value is applied to the control inductor and thepredetermined mutual inductance is generated in the inductor for theoscillator, when a circuit comprising the inductor for the oscillator isvaried, a desired inductance cannot be provided.

It is an object of the present invention to provide a inductancevariable device in which an inductance can be sequentially and largelyvaried and a desired inductance can be set even when a circuitcomprising an inductor is varied.

A inductance variable device according to the present invention ischaracterized in that the inductance variable device includes:

a first inductor;

a second inductor magnetically coupled to the first inductor;

a current source whose current is variable, electrically connected tothe first inductor; and

a current control unit operable to control a current flowing from thecurrent source to the first inductor according to a feedback signalhaving frequency information in a current flowing in the second inductorto vary a combined inductance of the second inductor.

In addition, the current control unit may include:

a frequency divider for dividing a frequency of the feedback signal to apredetermined frequency;

a phase detector for comparing a phase of the frequency divided signalwith a phase of a reference signal having predetermined frequencyinformation; and

a charge pump for outputting a voltage to the current source accordingto an input from the phase detector. In this case, the current controlunit can adjust a current value to be applied to the first inductor,according to the input signal from the charge pump.

Furthermore, the current source may include:

a plurality of coarse adjustment current sources operable to coarselyadjust a current value, in which each coarse adjustment current sourceis set to a different predetermined coarse adjustment current value; and

a fine adjustment current source operable to finely adjust a currentvalue according to an input signal from the charge pump.

In addition, the frequency divider may include:

a prescaler for dividing a frequency of an input signal by any ofseveral ratios; and

a counter for dividing a frequency of an input signal by a predeterminedratio.

Furthermore, the above inductance variable device may include a switchprovided between the first inductor and the current source. The switchcan connect the first inductor with the current source and disconnectthe first inductor from the current source.

Still furthermore, the first and second inductors may be formed in asemiconductor integrated circuit on a semiconductor substrate. Thesefirst and second inductors may be spirally formed of a strip-shapedconductor having one or more spirals.

In addition, the current source may be a DC power source. The currentsource may convert a direction of a direct current to be applied to thefirst inductor.

In addition, the current source may be an AC power source.

An IC chip according to the present invention is characterized in thatthe IC chip includes:

a first inductor provided on a semiconductor substrate;

a second inductor provided on the semiconductor substrate so as to bemagnetically coupled to the first inductor;

a current source whose current is variable, electrically connected tothe first inductor;

a frequency divider for dividing a frequency of a feedback signal havingfrequency information in a current flowing in the second inductor to apredetermined frequency;

a phase detector for comparing a phase of the frequency divided signalwith a phase of a reference signal having predetermined frequencyinformation; and

a charge pump for outputting a voltage to the current source accordingto an input from the phase detector, wherein

-   -   the first and second inductors, the current source, the        frequency divider, the phase detector, and the charge pump are        constituted as a semiconductor integrated circuit. Then, the        current source is adjusted according to an input signal from the        charge pump.

In addition, the current source may includes:

a plurality of coarse adjustment current sources operable to coarselyadjust a current value, in which each coarse adjustment current sourceis set to a different predetermined coarse adjustment current value; and

a fine adjustment current source operable to finely adjust a currentvalue according to an input signal from the charge pump.

Furthermore, the frequency divider may includes:

a prescaler for dividing a frequency of an input signal by any ofseveral ratios; and

a counter for dividing a frequency of an input signal by a predeterminedratio.

According to the present invention, a method of varying inductance in ainductance variable device having a first inductor, a second inductorprovided so as to be magnetically coupled to the first inductor, and acurrent source whose current is variable and that is electricallyconnected to the first inductor, includes:

(a) applying a predetermined current from the current source to thefirst inductor;

-   -   (b) dividing a frequency of a feedback signal having frequency        information in a current flowing in the second inductor so as to        be able to be compared with a reference signal;

(c) comparing a phase of the signal whose frequency was divided with aphase of the reference signal and outputting a difference signalcorresponding to its difference; and

(d) finely adjusting a current value to be applied to the first inductorby a voltage corresponding to the difference signal, to control thecurrent value to be applied to the first inductor by repeating the abovesteps (b) to (d) so that the difference between the frequency dividedsignal so that the reference signal becomes small and a combinedinductance in the second inductor is set to a predetermined value.

According to the present invention, by applying a current to the firstinductor which is one of the two inductors magnetically coupled to eachother and then changing the current value, the inductance of the secondinductor, which is the other of the two inductors, can be varied. Inthis case, the current to be applied to the first inductor is finelyadjusted according to a feedback signal having frequency information inthe current flowing in the second inductor, and thus a combinedinductance in the second inductor can be set to a predetermined value.By this, a resonance frequency in a resonance circuit including thesecond inductor can be set to various resonance frequencies such as 800MHz, 1.7 GHz, 2.1 GHz for a mobile phone, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become readily understood from the followingdescription of preferred embodiments thereof made with reference to theaccompanying drawings, in which like parts are designated by likereference numeral and in which:

FIG. 1 is a block diagram showing a constitution of a inductancevariable device according to a first embodiment of the presentinvention;

FIG. 2 is a block diagram showing a constitution of a prescaler shown inFIG. 1 in detail;

FIG. 3 is a block diagram showing a constitution of a current sourceshown in FIG. 1 in detail;

FIG. 4 is a plan view showing a inductance variable device according toa second embodiment of the present invention;

FIG. 5 is a sectional view taken along line II-II in FIG. 4;

FIG. 6A shows an equivalent circuit of the inductance variable deviceaccording to the present invention when the current source is a DC powersource;

FIG. 6B shows an equivalent circuit of the inductance variable deviceaccording to the present invention when the current source is an ACpower source;

FIG. 7 shows an equivalent circuit of a inductance variable deviceaccording to a third embodiment of the present invention;

FIG. 8 is a plan view showing a conventional inductor device accordingto a first example;

FIG. 9 is a plan view showing a conventional inductor device accordingto a second example; and

FIG. 10 is a plan view showing a conventional inductor device accordingto a third example.

PREFERRED EMBODIMENT OF THE PRESENT INVENTION

Preferred embodiments of a inductance variable device according to thepresent invention will be described with reference to the accompanieddrawings hereinafter. In addition, the same reference numerals areallotted to the same member essentially in the drawings.

First Embodiment

FIG. 1 is a block diagram showing a constitution of a inductancevariable device 10 according to a first embodiment of the presentinvention. The inductance variable device 10 includes a first inductor16, a second inductor 17 arranged so that it can be coupled to the firstinductor 16 magnetically, a current source 4 applying a current to thefirst inductor 16, a charge pump (CP) 6, a phase detector (PD) 7, acounter 8 and a prescaler 9. The first and second inductors 16 and 17may be provided on a semiconductor substrate. In this case, the firstand second inductors 16 and 17 may be stacked. In addition, the firstand second inductors 16 and 17, the current source 4, the charge pump(CP) 6, the phase detector (PD) 7, the counter 8 and the prescaler 9 maybe constituted as a semiconductor integrated circuit. By thus, theinductance variable device 10 can be constituted as an IC chip.

FIG. 2 is a block diagram showing a constitution of the prescaler 9 indetail. In the prescaler 9, a frequency can be divided in a fixedfrequency division ratio 32/33, 16/17 or 8/9 according to a digitalsignal from a base band (BB) depending on a desired frequency of aninput signal. In addition, the counter 8 can divide the frequency to1/20. In addition, a frequency divider consists of the prescaler 9 andthe counter 8.

The phase detector (PD) 7 compares a phase of a reference signalfrequency of 5 MHz with a phase of a frequency of the signal divided bythe divider including the prescaler 9 and the counter 8 and outputs asignal corresponding to a phase difference. The charge pump 6 outputs avoltage signal for fine adjustment to the current source 4,corresponding to the phase difference signal outputted from the phasedetector 7.

FIG. 3 is a block diagram showing a constitution of the current source 4in detail. The current source 4 includes coarse adjustment currentsources 11 and a fine adjustment current source 12. Here, three kinds ofcurrent sources are shown as the coarse adjustment current source 11.The current source 4 switches the coarse adjustment current sources 11having different current abilities corresponding to the desiredfrequencies of the input signals, according to a digital signal from thebase band. In addition, it adjusts the fine adjustment current source 12by a voltage outputted from the charge pump 6, according to a feedbacksignal from the second inductor 17.

In addition, a combined inductance L_(total) which is generated in thesecond inductor 17 by a current value i₁ applied to the first inductor16 is provided from the following equation.$L_{total} = {L_{2} + {M\frac{i_{1}}{i_{2}}}}$

Here, a character L₂ designates a self-inductance of the second inductor17. A character M designates a mutual inductance of the first and secondinductors 16 and 17. In addition, reference character i₁ designates acurrent value to be applied to the first inductor 16. Furthermore, i₂designates a current value flowing in the second inductor 17.

According to the above equation, it is thought that when a predeterminedcurrent value i₁ is applied to the first inductor 16, the desiredcombined inductance L_(total) can be provided in the second inductor 17.However, as the current value i₂, and such, flowing in the secondinductor 17 varies, there are cases in which the desired combinedinductance L_(total) cannot be provided with the predetermined currentvalue provided from the coarse adjustment current source 11. Thus,according to the inductance variable device 10, the current value i₁ tobe applied to the first inductor 16 is finely adjusted by the feedbacksignal from the second inductor 17 so that the desired L_(total) can beprovided in the second inductor 17.

Next, an operation of the inductance variable device 10 will bedescribed hereinafter. According to the inductance variable device 10,the combined inductance L_(total) in the second inductor 17 can be setto a predetermined value when the current value to be applied to thefirst inductor 16 is set to a predetermined value. Thus, a resonancefrequency of a resonance circuit including the second inductor 17 can beset to a predetermined value. According to the operation of theinductance variable device 10, the current value is coarsely adjustedfirst and then finely adjusted according to the signal from the secondinductor 17.

(a) A signal having RF frequency information and the like is inputted tothe base band (BB) IC through an external amplifier and the like.

(b) The frequency information from the base band is inputted to theinductance variable device 10 in the form of a digital signal.

(c) The coarse adjustment current source 11 in the current source 4 isselected according to the inputted frequency information, and a currentis applied from the selected coarse adjustment current source 11 to thefirst inductor 16 (coarse adjustment). According to this coarseadjustment, a current value to be applied to the first inductor 16 ispreviously calculated and the coarse adjustment current sources 11 whichwere set to respective coarse adjustment current values are provided, sothat the resonance frequency in the resonance circuit including thesecond inductor 17 can be set to resonance frequencies 800 MHz, 1.7 GHz,2.1 GHz and the like for a mobile phone. Thus, the coarse adjustmentcurrent source 11 corresponding to the frequency information can beselected.

(d) In addition, the frequency to be divided by the prescaler 9 isselected according to the inputted frequency information.

(e) Then, the signal (feedback signal) from the second inductor 17 isinputted to the prescaler 9. The frequency of the inputted signal isdivided so as to be able to be compared with the reference signal by theprescaler 9 and the counter 8. In addition, signal intensity reaches anacceptable value of input sensitivity of the prescaler 9 by the abovecoarse adjustment.

(f) The phase detector 7 compares a phase of the signal whose frequencywas divided with a phase of the reference signal and its differencesignal is outputted to the charge pump 6.

(g) The charge pump 6 outputs a voltage corresponding to the inputteddifference signal to the fine adjustment current source 12 of thecurrent source 4.

(h) The fine adjustment current source 12 finely adjusts the currentvalue to be applied to the first inductor 16 by the voltage from thecharge pump 6.

(i) The current value to be applied to the first inductor 16 iscontrolled so that a difference between the signal whose frequency wasdivided and the reference signal may converge, that is, become small byrepeating the above steps (e) to (h), whereby the combined inductanceL_(total) in the second inductor 17 is set to the predetermined value.Thus, the resonance frequency of the resonance circuit including thesecond inductor 17 can be set to the predetermined resonance frequency,for example. More specifically, it can be set to various resonancefrequencies such as 800 MHz, 1.7 GHz, 2.1 GHz for a mobile phone.

As described above, according to the inductance variable device 10, thecombined inductance L_(total) in the second inductor 17 can be set tothe predetermined value by finely adjusting the current value aftercoarsely adjusting the current value to be applied to the first inductor16 and having the signal from the second inductor 17 feed back.

Second Embodiment

A inductance variable device according to a second embodiment of thepresent invention will be described with reference to FIGS. 4 to 6.

FIG. 4 is a plan view showing the inductance variable device accordingto the second embodiment of the present invention. FIG. 5 is a sectionalview taken along line II-II in FIG. 4. In the plan view in FIG. 4, aconductor is hatched, and in the sectional view in FIG. 5, only theconductor is hatched to make the drawings clear. In FIGS. 4 and 5, aninsulation film 19 is formed on a surface of a substrate 21 such as anonmagnetic substrate or a semiconductor substrate, and a first inductor16 which is a strip-shaped conductor film and spirally formed into arectangle (square, for example) is formed on the insulation film 19. Aninsulation film 22 is provided on the first inductor 16 and a secondinductor 17 which is also a strip-shaped conduction film having the samewidth as that of the first inductor 16 and spirally formed into arectangle so as to overlap with the first inductor 16 on the insulationfilm 22. An insulation film 23 is formed on the second inductor 17. Aninner end 16 a of the first inductor 16 is connected to a lead conductor25 a provided on a surface of the insulation film 23 through a conductorpost 16 e which penetrates the insulation films 22 and 23 so as not tocome in contact with an inner end 17 a of the second inductor 17. Theinner end 17 a of the second inductor 17 is connected to a leadconductor 26 a through a conductor post 17 e which penetrates theinsulation film 23. In addition, a part having the first inductor 16,the first insulation film 22, the second inductor 17, the secondinsulation film 23, and the first and second lead conductors 25 a and 26a in the inductance variable device shown in FIG. 5 may be verticallyreversed and formed on the insulation film 19. In this structure, theinductance variable device can be appropriately mounted on a substratesuch as an IC substrate.

Similarly, an outer end 16 b of the first inductor 16 is connected to alead conductor 25 b through a conductor post (not shown). An outer end17 b of the second inductor 17 is connected to a lead conductor 26 bthrough a conductor post (not shown). A DC power supply 4 is connectedbetween the lead conductors 25 a and 26 b. A terminal 14 of the leadconductor 26 a and a terminal 15 of the lead conductor 26 b areterminals to connect the second inductor 17 to an external circuit. Asize of the first and second inductors 16 and 17 is 200 μm square, forexample, and when the inductance variable device according to thepresent invention is incorporated in an integrated circuit, a width ofthe strip-shaped conductor is about 10 μm. The configuration of thefirst and second inductors 16 and 17 is not limited to a spiralrectangle, it may be other configurations such as a circle, a hexagon,an octagon.

The DC power supply 4 a is provided to apply a predetermined directcurrent to the first inductor 16 and a current value and a currentdirection in the first inductor 16 can be controlled by a currentcontrol circuit 5. When the current value of the first inductor 16 isvaried, a magnetic flux density of the first inductor 16 can be variedand the inductance of the second inductor 17 which is magneticallycoupled to the first inductor 16 can be varied.

FIG. 6A is an equivalent circuit of the inductance variable device shownin FIG. 4. The first inductor 16 is magnetically coupled to the secondinductor 17 and both ends of the first inductor 16 are connected to bothterminals of the DC power supply 4 a. The second inductor 17 has bothterminals 14 and 15. The current control circuit 5 has an input terminal20. Thus, the current control circuit 5 receives a control signal fromthe outside by the input terminal 20 and it can vary a direction of acurrent and a value thereof applied from the DC power supply 4 a to thefirst inductor 16.

FIG. 6B is an equivalent circuit of the inductor device when analternate current is applied to the first inductor 16. An AC powersupply 4 b is connected to the first inductor 16 as a current source.The alternate current is induced in the second inductor 17 and itsinductance is varied by the alternate current flowing in the firstinductor 16.

Third Embodiment

FIG. 7 is an equivalent circuit of a inductance variable deviceaccording to a third embodiment of the present invention. Referring toFIG. 7, a first inductor 16, a second inductor 17, a DC power supply 4 aand a current control circuit 5 have the same constitution of those inthe second embodiment. According to the inductance variable device shownin FIG. 7, a switch 27 including a semiconductor element is providedbetween the first inductor 16 and the DC power supply 4 a. When acontrol input is applied to a control terminal 28 of the switch 27, theswitch 27 is closed.

According to the inductance variable device shown in FIG. 7, when thesame operation as that of the inductance variable device shown in FIG. 4is performed, the switch 27 is closed and a direct current is applied tothe first inductor 16. When the switch 27 is opened and the directcurrent is not applied to the first inductor 16, the second inductor 17becomes an inductor having a constant inductance.

The present invention may also have the following constitution shown invarious embodiments.

According to a first constitution, the inductance variable deviceaccording to the present invention includes a first inductor, a secondinductor magnetically coupled to the first inductor, and a currentsource whose current is variable, electrically connected to the firstinductor.

According to the present invention, when a current to be applied to thefirst inductor is varied, a magnetic flux of the second inductor whichis magnetically coupled to the first inductor is varied. By the magneticflux being varied, the inductance of the second inductor is varied. Whena current value and a current direction to be applied to the firstinductor is varied, the inductance variable device can vary itsinductance over a large range.

According to a second constitution, the inductance variable deviceaccording to the present invention is characterized in that theinductance variable device includes:

a nonmagnetic substrate having a first insulation film on one surface;

a first inductor in which a strip-shaped conductor layer is spirallyformed on the first insulation film;

a second insulation film covering the first inductor;

a second inductor in which a conductor layer is formed on the firstinductor through the second insulation film;

a third insulation film formed on the second inductor;

a pair of first lead conductors penetrating the second and thirdinsulation films, in which respective one ends are connected to bothends of the first inductor and respective other ends are led to asurface of the third insulation film;

a pair of second lead conductors for penetrating the third insulationfilm, in which respective one ends are connected to both ends of thesecond inductor and respective other ends are led to a surface of thethird insulation film; and

a current source whose current is variable, electrically connected tothe pair of first lead conductors.

According to the present invention, it is possible to constitute theinductance variable device capable of varying its inductance from theconductor layers and the insulation films formed on the nonmagneticsubstrate. Since the conductor layer and the insulation film can beformed on the same substrate on which the semiconductor element isformed in a process of manufacturing a semiconductor integrated circuit,the manufacturing process of the inductance variable device becomessimple, and therefore it is possible to reduce the size and cost of theinductance variable devices.

Although the present invention has been detailed in the above mode forcarrying out the invention with reference to the above embodiments, thepresent invention is not limited to the above-described embodiments. Itis obvious for those skilled in the art that various modifications andvariations that are preferred may be included in the scope of thepresent invention described in the claims below.

1. A inductance variable device comprising: a first inductor; a secondinductor magnetically coupled to the first inductor; a current sourcewhose current is variable, electrically connected to the first inductor;and a current control unit which controls a current flowing from thecurrent source to the first inductor according to a feedback signalhaving frequency information in a current flowing in the second inductorto vary a combined inductance of the second inductor.
 2. The inductancevariable device according to claim 1, wherein the current control unitcomprises: a frequency divider for dividing a frequency of the feedbacksignal to a predetermined frequency; a phase detector for comparing aphase of the frequency divided signal with a phase of a reference signalhaving predetermined frequency information; and a charge pump foroutputting a voltage to the current source according to an input fromthe phase detector, and thereby the current control unit adjusts acurrent value to be applied to the first inductor according to an inputsignal from the charge pump.
 3. The inductance variable device accordingto claim 2, wherein the current source comprises: a plurality of coarseadjustment current sources operable to coarsely adjust a current value,wherein each coarse adjustment current source is set to a differentpredetermined coarse adjustment current value; and a fine adjustmentcurrent source operable to finely adjust the current value according toan input signal from the charge pump.
 4. The inductance variable deviceaccording to claim 2, wherein the frequency divider comprises: aprescaler for dividing a frequency of an input signal by any of severalratios; and a counter for dividing a frequency of an input signal by apredetermined ratio.
 5. The inductance variable device according toclaim 1, further comprising: a switch provided between the firstinductor and the current source, wherein the switch connects the firstinductor with the current source or disconnects the first inductor fromthe current source.
 6. The inductance variable device according to claim1, wherein the first and second inductors are formed in a semiconductorintegrated circuit on a semiconductor substrate.
 7. The inductancevariable device according to claim 1, wherein the first and secondinductors are spirally formed of a strip-shaped conductor having one ormore spirals.
 8. The inductance variable device according to claim 1,wherein the current source is a DC power source.
 9. The inductancevariable device according to claim 8, wherein the current source canconvert a direction of a direct current to be applied to the firstinductor.
 10. The inductance variable device according to claim 1,wherein the current source is an AC power source.
 11. An IC chipcomprising: a first inductor provided on a semiconductor substrate; asecond inductor provided on the semiconductor substrate so as to bemagnetically coupled to the first inductor; a current source whosecurrent is variable, electrically connected to the first inductor; afrequency divider for dividing a frequency of a feedback signal havingfrequency information in a current flowing in the second inductor to apredetermined frequency; a phase detector for comparing a phase of thefrequency divided signal with a phase of a reference signal havingpredetermined frequency information; and a charge pump for outputting avoltage to the current source according to an input from the phasedetector, wherein the first and second inductors, the current source,the frequency divider, the phase detector, and the charge pump areconstituted as a semiconductor integrated circuit, thereby the currentsource adjusts the current value according to an input signal from thecharge pump.
 12. The IC chip according to claim 11, wherein the currentsource comprises: a plurality of coarse adjustment current sourcesoperable to coarsely adjust a current value, wherein each coarseadjustment current source is set to a different predetermined coarseadjustment current value; and a fine adjustment current source operableto finely adjust the current value according to an input signal from thecharge pump.
 13. The IC chip according to claim 12, wherein thefrequency divider comprises: a prescaler for dividing a frequency of aninput signal by any of several ratios; and a counter for dividing afrequency of an input signal by a predetermined ratio.
 14. A method ofvarying inductance in a inductance variable device having a firstinductor, a second inductor provided so as to be magnetically coupled tothe first inductor, and a current source whose current is variable andthat is electrically connected to the first inductor, the methodcomprising: (a) applying a predetermined current from the current sourceto the first inductor; (b) dividing a frequency of a feedback signalhaving frequency information in a current flowing in the second inductorso as to be able to be compared with a reference signal; (c) comparing aphase of the signal whose frequency was divided with a phase of thereference signal and outputting a difference signal corresponding to itsdifference; and (d) finely adjusting a current value to be applied tothe first inductor by a voltage corresponding to the difference signal,to control the current value to be applied to the first inductor byrepeating the above steps (b) to (d) so that the difference between thefrequency divided signal and the reference signal becomes small, therebya combined inductance in the second inductor is set to a predeterminedvalue.